It is known that most current airplanes including airliners, are provided with an traffic function ATSAW (Airborne Traffic Situational AWareness), allowing to determine, for a given airplane, information regarding the air traffic around the airplane from data exchanged with surrounding airplanes.
The data to be exchanged essentially originate from communication systems ADS-B (Automatic Dependant Surveillance-Broadcast), but also from on-board collision avoidance systems TCAS (Traffic alert and Collision Avoidance System).
The information from surrounding aircrafts (relative position, altitude, course, velocity, identification, etc.) can be displayed in the cockpit of the airplane, either graphically on a navigation screen, or textually on a multifunction control and display device.
From such information, the crew of an airplane could decide to perform (through the ATSAW function) an altitude change towards a more appropriate altitude level, within an air space without any radar coverage (either controlled or not), according to an in-travel altitude change ITP (In Trail Procedure) within reduced spacings, as defined and regulated (velocity, separation distance, etc.) by the International Civil Aviation Organisation (or OACI). The ITP procedure is particularly detailed in <<Safety, Performance and Interoperability Requirements Document for ATSA-ITP Application>> published by the EUROCAE (ED159) and the RTCA (D0 312).
An altitude change within reduced spacings in accordance with the ITP procedure, taking into account standards of minimum separation between airplanes in times and (horizontal and vertical) distances enforced by the OACI, is controlled and monitored by controlling the air traffic, in charge of meeting such standards.
Such an altitude change allows particularly:                to reduce fuel consumption of the airplane while reaching an optimum cruising altitude level and, consequently, to reduce the emissions of polluting gas;        to improve the flight quality and safety, for instance while reaching altitude levels with more favorable winds on the trajectory being followed;        to indirectly increase the traffic on the air space in question (an airplane releasing its current altitude level offers its place to another airplane);        etc.        
For performing an altitude change within reduced spacings in accordance to the rules in force, the crew of an airplane preliminarily observes the air traffic around the airplane (on the navigation screen and/or on the multifunction control and display device), then, if he judges that the altitude change is feasible, he submits a clearance request to the air traffic control. After checking and meeting the separation standards between airplanes in the given air space, the air traffic controller grants a clearance or not.
However, should clearance be denied, the crew is not aware of the origin of such a denial (for instance, the aircraft(s) preventing the altitude change maneuver). They will be then able to reiterate their clearance request later on, without being more certain to get it, namely because their request relies on a vague and inaccurate observation of the surrounding air traffic. Also, performing an altitude change maneuver within reduced spacings currently remains very random.